This application claims priority from European Application No. 00 106 612.5, filed on Mar. 28, 2000, and European Application No. 00 106 687.7, filed on Mar. 29, 2000, the subject matter of each of which is hereby incorporated herein by reference.
1. Field of the Invention
The invention relates to doped titanium dioxide, to a process for its preparation and to its use.
2. Background Information
Pyrogenic titanium dioxide (obtainable commercially as Degussa TiO2 P 25) is distinguished by the variety of its possible uses in the field of photocatalysis.
(R. W. Matthews, S. R. McEvoy, J. Photochem. Photobiol. A: Chem., 64 (1992) 231-246.
R. I. Bickley et al., Journal of Solid State Chemistry, 92 (1991), 178-190.
R. Franke, C. Franke, Chemosphere, Vol. 39, No. 15 (1999), 2651-2659.
H. Zen, JETI (1998), 46 (10), 66-67.
It is used as a reference material having a high degree of photocatalytic activity.
(V. Loddo et al., Applied Catalysis B: Environmental 20 (1999), 29-45).
The invention provides a titanium dioxide doped by means of an aerosol and containing an oxide from the group zinc oxide, platinum oxide, magnesium oxide and/or aluminium oxide as the doping component.
The invention also provides a process for the preparation of the titanium dioxide doped by means of an aerosol, which process is characterised in that an aerosol is fed into a flame such as is used for the preparation of pyrogenic titanium dioxide by means of flame hydrolysis, that aerosol is mixed homogeneously before or during the reaction with the gas mixture of the flame oxidation or flame hydrolysis, the aerosol/gas mixture is allowed to react in a flame, and the resulting doped, pyrogenically prepared oxide is separated from the gas stream in a known manner, there being used as the starting material for the aerosol a salt solution or suspension containing the component of the substance to be doped, which may be a metal salt or metalloid salt or mixtures of the two or a suspension of an insoluble metal or metalloid compound or a mixture of the two, the aerosol being produced by atomisation by means of a two-component nozzle or by an aerosol generator, preferably by ultrasonic atomisation.
There may be used as the substance to be doped salts of zinc, magnesium, aluminium and/or noble metals such as platinum, palladium, silver, gold. There may preferably be used aqueous solutions of those salts, which may optionally be acidified. There may preferably be used as salts zinc chloride, hexachloroplatinic acid, magnesium chloride, aluminium chloride.
The process for doping by means of an aerosol may be carried out substantially as described in the document EP 0 850 876 A1.
The process of flame hydrolysis to prepare pyrogenic titanium dioxide is known from Ullmann""s Enzyklopxc3xa4die der technischen Chemie, 4th edition, Volume 21, page 464.
The titanium dioxides doped by means of an aerosol according to the invention may exhibit concentrations of the doping substances in a range of from 0.00001 to 20 wt. %., preferably from 0.1 to 10,000 ppm. The BET surface areas may be from 5 to 150 m2/g, preferably from 35 to 110 m2/g.
In order to produce a high level of photocatalytic activity, the BET surface area may be from 65 to 80 m2/g. In that case, the amount of doping component may be from 40 to 800 ppm.
In order to produce low photocatalytic activity, the BET surface area may be from 35 to 60 m2/g. In that case, the amount of doping component may be greater than 1000 ppm.
When the titanium dioxides doped by means of an aerosol according to the invention have a high level of photocatalytic activity, they may be used for the purification of waste air.
They may be fixed to a support.
When the titanium dioxides according to the invention have a high level of photocatalytic activity, they may be used for the degradation of impurities in waste water and/or waste air. In that case, the titanium dioxides may be used both suspended in the waste water and/or waste air and fixed to a support.
When the titanium dioxides according to the invention have low photocatalytic activity, they may be used as an adsorbent for UV radiation. They may be used in the coating of glasses or in plastics.
The titanium dioxides according to the invention may also be used for application to glasses, to plastics, for the removal of impurities from air, water, etc. when they have a high level of photocatalytic activity.
The titanium dioxides according to the invention having a high level of photocatalytic activity may also be used for the sterilisation of water with UV irradiation.
The photocatalytic activity of the titanium dioxides doped by means of an aerosol according to the invention is tested in the photocatalytic degradation of chlorinated hydrocarbons with UV irradiation in optionally acidified, aqueous suspension.
In those tests, it is found that the photocatalytic activity of the titanium dioxides according to the invention in optionally acidified aqueous suspension can be increased or reduced by doping with oxides of metals/noble metals or metalloids.
Surprisingly, the photocatalytic activity in the degradation of chlorinated hydrocarbons in aqueous suspension is increased even though homogeneous intermixing of the doping component and the titanium dioxide has taken place. The doping component in that case is therefore not exclusively on the titanium dioxide, but also in the titanium dioxide.
A higher degree of doping contributes towards lowering the photocatalytic activity. In order to determine the photocatalytic activity, the degradation of chlorinated hydrocarbons (4-chlorophenol (4-CP) and dichloroacetic acid (DCA)) with UV irradiation in a stirred reactor is tested.
In order to increase the rate of photocatalytic degradation of chlorinated hydrocarbons in optionally acidic aqueous suspension by the doped pyrogenically prepared titanium dioxides, the BET surface area is preferably in the range of from 70 to 85 m2/g.
In order to lower the photocatalytic activity, which is likewise tested by the degradation of 4-chlorophenol and dichloroacetic acid with UV irradiation in purely aqueous or acidified aqueous suspension, the BET surface area is preferably in the range of from 50 to 60 m2/g.
Moreover, a change in the amount of doping component leads to a change in the rate of photocatalytic degradation of the chlorinated hydrocarbons with UV irradiation.